President Barack Obama's efforts to persuade the U.S. Congress to back his plan to attack Syria were met with skepticism on Monday from lawmakers in his own Democratic Party who expressed concern the United States would be dragged into a new Middle East conflict.

"There is a lot of skepticism," said Representative Jim Moran after taking part in a 70-minute phone briefing for Democratic lawmakers by Obama's top national security aides about the response to a chemical weapons attack that U.S. officials say killed 1,429 people on the outskirts of Damascus.

Obama appeared to make some headway, however, with two influential Republican senators, John McCain and Lindsey Graham, who came out of a White House meeting convinced that Obama is willing to use air strikes not just to destroy Syrian chemical weapons capability but also to bolster Syrian rebels.

McCain, long an advocate of a more robust U.S. approach to Syria, said failure to get behind strikes against President Bashar al-Assad's forces would be "catastrophic."

While Obama faced obstacles at home, key U.S. ally France said it had evidence showing that Assad's government had ordered chemical attacks and was determined to punish him.

The French government released a nine-page intelligence document that listed five points that suggested Assad's fighters were behind the "massive and coordinated" August 21 chemical attack.

Assad, in an interview with French newspaper Le Figaro, warned the French to steer clear of policies hostile to Syrians or else, "the state will be their enemy."

Obama's abrupt decision to halt plans for a strike against Assad's forces and instead wait for congressional approval has generated a raging debate just as the president prepares to depart on Tuesday on a three-day trip to Sweden and Russia.

The biggest obstacle he faces is winning the support of members of his own party in the House of Representatives and conservative Republicans who see little need for the United States to get involved in distant civil wars.

Secretary of State John Kerry, who was among the Obama advisers on the call for the Democrats, urged support for giving Obama a resolution to use force, saying Syria had reached a "Munich moment," according to participants.

This was a reference to 1938 Munich Agreement seen as appeasement by Britain and France to Adolf Hitler before World War Two.

The White House argument is that Syria must be punished for the chemical weapons onslaught and that at stake are the integrity of an international ban on such weapons and the need to safeguard U.S. national security interests and allies Israel, Jordan and Turkey.

Syria has blamed the attack on rebel forces.

Samples linked to the August 21 attack were shipped today to European labs and were due to arrive within hours, a U.N. statement said. U.N. chemical weapons experts visited the site of the attack last week.

On Tuesday, U.N. disarmament chief Angela Kane, who has just returned from Syria, will update more than 30 countries that co-signed a British letter asking the United Nations to investigate the attack, the statement said.

DEMOCRATS WARY

As with much in which the divided U.S. Congress involves itself, there was deep disagreement on how to proceed.

Representative Chris Van Hollen, a senior Democrat and an ally of Obama on many issues, complained that the wording of the White House's request to Congress for the authorization of the use of force was too open-ended and could lead to deep U.S. involvement in Syria, where more than 100,000 people have been killed in more than two years of fighting.

"There is no limitation on putting American soldiers on the ground. There is no end point" on the resolution, he said. "The draft resolution presented by the administration is overly broad, it provides too much of a blank check to the executive."

White House officials said they are willing to make changes to the draft resolution authorizing force in order to address lawmakers' concerns.

Overshadowing the debate are the ghosts of wars in Iraq and Afghanistan, conflicts that lasted far longer and were far more expensive than first predicted. Congressional hesitancy reflects the overall weariness of war among Americans, who oppose getting involved in Syria.

"My guess is if the vote was taken tomorrow, the president would lose or get a resolution so watered down that he would be better off without one," Democrat Moran said.

Obama needs to shore up his left wing, particularly in the House, where both liberal Democrats and many conservative "Tea Party" Republicans are opposed to more U.S. military action in the Middle East.

McCain and Graham are from the more traditional wing of the Republican Party that generally favors U.S. intervention abroad when deemed necessary. They want a broad strategy not just to punish Assad for the chemical weapons attack but to help Syria's rebels.

Speaking to reporters, McCain and Graham gave dire warnings about the impact if Congress fails to support Obama. Both blamed Obama for not making the case for Syrian intervention sooner.

"If the Congress were to reject a resolution like this after the president of the United States has already committed to action, the consequences would be catastrophic," McCain said.

He said Obama appeared to be planning more than retaliatory strikes.

"We have been given some reason to believe that very serious air strikes may take place as opposed to cosmetic," said McCain. Added Graham: "For the first time I've gotten an understanding today of what happens after the smoke clears."

The administration will bring out two of its heavy guns on Tuesday to try to persuade Congress on Syria when Kerry and Defense Secretary Chuck Hagel address the Senate Foreign Relations Committee.

The Democratic-led Senate is expected to approve U.S. military action but there is stronger opposition in the House.

Representative James McGovern, a liberal Democrat from Massachusetts, said he was skeptical that Obama's plan for military strikes could help end the war in Syria. If the vote were taken today, he said, he would vote no.

"People are horrified by the pictures of people suffering and they genuinely want to help. But people have become, it's more than just war-weary, they've become skeptical of the effectiveness of these military involvements," he said.

With Navy ships in place and ready to launch cruise missiles on Obama's order, no decision was likely until days after Congress returns from its summer recess on September 9. In the interim, Obama is using the time to build his case.

Washington's hesitancy has prompted mocking comments from Syrian leaders and a push from Assad's chief backer, Russia, to send members of the Russian parliament to the U.S. capital to argue against a strike.

Obama's gamble to seek congressional backing carries many risks, chief among them is that Congress will again thwart him and make him look weak around the world.

Obama may have to build a majority House vote based on his fellow Democrats joining those Republicans who support action, a senior House Republican aide told Reuters on Sunday.

"It's too early to speculate" what the House will do, the aide said, but House Democratic leader Nancy Pelosi "is going to have to post a big number" among her members in support of it. The aide declined to predict how big.

Republicans hold the House, 233-200, with two vacancies.

It is not clear whether House Speaker John Boehner, a Republican, is willing or able to back Obama's plan. His members have defied him in the past when he has tried to strike compromise with Obama on budget issues.